Manchester United boss Louis van Gaal admits Wayne Rooney may have made the knee injury that is set to sideline the striker for six weeks worse by playing through the pain last weekend.

Van Gaal said at his press conference ahead of Thursday's Europa League clash with FC Midtjylland that the reason Rooney had not travelled with United to Denmark was a knee problem sustained in the 2-1 Barclays Premier League defeat at Sunderland on Saturday.

The manager would not put a timeframe on Rooney's absence, but Post Sport understands the Red Devils and England captain is facing around six weeks out.

It was then put to Van Gaal that Rooney had played the full duration of Saturday's game rather than coming off, and the Dutchman said: "That's typical Wayne.

"He's a guy who wants to go until the end - if he's feeling pain he doesn't want to go off. He wants to win.

"That's a fantastic attitude but sometimes it's also bad for his body.

"But you cannot say (if he made the injury worse by staying on) because he didn't know when it happened. That's the difficulty."

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Van Gaal stressed United will simply "have to cope" after an injury that takes the club's list of unavailable senior players to 12.

But the loss of Rooney - as well as being a worry for England as they prepare for Euro 2016 - would appear a major setback for Van Gaal at a time when his team are struggling in the race for a top-four finish in the Premier League and the 64-year-old's position is under increasing scrutiny.

Rooney, with 14 goals in all competitions, is United's top scorer this season and the 30-year-old has netted seven times in his last nine matches.

While making reference to the options he does have available up front - the key one being Anthony Martial - Van Gaal did mention Will Keane, who is among several inexperienced players in the squad for Thursday's last-32 first leg, along with Donald Love, Regan Poole, Joe Riley and James Weir.

Van Gaal also reiterated that winning the Europa League is now looking United's most viable route into the Champions League.

But he said of Danish champions Midtjylland, who defeated Southampton in the Europa League play-offs in August: "We must not forget we have to beat a very good team.

"And the circumstances are also not so easy - the pitch is smaller than usual. So it will not be so easy to beat Midtjylland here."